ViewDragHelper实现相册图片查看

ViewDragHelper实现相册中图片滑动查看功能

ViewDragHelper是SupportV4提供的实用工具类，主要用于多视图下View的拖拽滑动效果，比如侧滑栏，抽屉都能实现。程序员无需再去处理繁琐的onTouch事件问题。

本文主要实现一个简单的图片滑动查看效果

实现过程

 - 初始化ViewDragHelper  - 复写ViewDragHelper.Callback接口方法  - 传递onTouch事件  - 编写业务逻辑 

 1. 初始化ViewDragHelper

   ViewDragHelper viewDragHelper;
    private void init() { //this 指parent ViewGroup实例引用
        viewDragHelper = ViewDragHelper.create(this, callback);
    }


 2. 实现Callback接口

 private ViewDragHelper.Callback callback = new ViewDragHelper.Callback() {

        手指捕获时可以得到当前是那个View，返回true表示可以被拖拽移动
        public boolean tryCaptureView(View child, int pointerId) {
            return false;
        }

        当某个View发生变化后 触发这个方法，可以获取到left,top新的值，以及变化量dx,dy
        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
            super.onViewPositionChanged(changedView, left, top, dx, dy);
        }

        当手指释放时触发这个方法，这里可以写动画处理，参数为x,y方向的速度
        public void onViewReleased(View releasedChild, float xvel, float yvel) {
            super.onViewReleased(releasedChild, xvel, yvel);
        }

        上下拖拽view top为新的位置，dy为变化量
        public int clampViewPositionVertical(View child, int top, int dy) {
            return top;
        }

        左右拖拽view left为新的位置 dx为变化量
        public int clampViewPositionHorizontal(View child, int left, int dx) {
            return left;
        }
  };

 3. 传递事件
   交给dragHelper去做
    public boolean onInterceptTouchEvent(android.view.MotionEvent ev) {
        return viewDragHelper.shouldInterceptTouchEvent(ev);
    };

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        viewDragHelper.processTouchEvent(event);
        return true;
    }

 4. 编写具体逻辑
 我们具体的业务基本都在回调函数中，我们的代码是扩展FrameLayout，下面的代码都放在子类中
 import android.support.v4.view.ViewCompat;
 import android.support.v4.widget.ViewDragHelper;
 import android.support.v4.widget.ViewDragHelper.Callback;
 public class SmartShowImageLayout extends FrameLayout {
   //代码都添加到这里
 }

 XML布局
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <com.gaofeng.mobile.smart.images.SmartShowImageLayout
        android:id = "@+id/layout"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_centerHorizontal="true" >

        <ImageView
            android:id="@+id/cachenext"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"

             />

        <ImageView
            android:id="@+id/next"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:src = "@drawable/m06"
             />

         <ImageView
            android:id="@+id/currentview"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" 
            android:layout_gravity="center"
            android:src="@drawable/m05" 
            />

    </com.gaofeng.mobile.smart.images.SmartShowImageLayout>

</RelativeLayout>

 首先创建实体保存必要的变量
    final class Holder {
        public ImageView imageView;//显示的View
        //记住以前的位置
        public int left;
        public int top;
        public int right;
        public int bottom;

        public void setPosition(int l, int t, int r, int b) {
            this.left = l;
            this.top = t;
            this.right = r;
            this.bottom = b;
        }

        public void setImageView(ImageView imageView) {
            this.imageView = imageView;
        }
    }

    Holder next, current, cachenext;//下一张，当前显示，cachenext未使用
    //初始化Holder

        public void initHolder() {
        Log.d(TAG, "initHolder");

        if (cachenext == null) { //本文中未用
            cachenext = new Holder();
            View view = getChildAt(0);
            cachenext.setPosition(view.getLeft(), view.getTop(),
                    view.getRight(), view.getBottom());
            cachenext.imageView = (ImageView) view;
        }

        if (next == null) {
            next = new Holder();
            View view = getChildAt(1);
            next.setPosition(view.getLeft(), view.getTop(), view.getRight(),
                    view.getBottom());
            next.imageView = (ImageView) view;
            // 设置默认的scale和alpha
            next.imageView.setScaleX(defScaleX);
            next.imageView.setScaleY(defScaleY);
            next.imageView.setAlpha(defAlpha);
        }

        if (current == null) {
            current = new Holder();
            View view = getChildAt(2);
            current.setPosition(view.getLeft(), view.getTop(), view.getRight(),
                    view.getBottom());
            current.imageView = (ImageView) view;
        }

    }
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        Log.d(TAG, "onLayout:" + changed);
        if (changed) {
            initHolder(); //布局完成放置，我们才能获取具体位置值
        }
    };

  //变量初始化
    boolean isSmooth = true;
    int direction; // 1 is Left, 2 is Right
    //初始化下一张的 scale和alpha 值，用来添加特效的
    float defScaleX = 0.2f;
    float defScaleY = 0.2f;
    float defAlpha = 0.2f;
    int screenWidth = 0;//屏幕宽度
    int screenHeight = 0;//屏幕高度
    int currentIndex = 0;
   //图片资源
  final static int images[] = { R.drawable.m03, R.drawable.m04,
            R.drawable.m05, R.drawable.m06 };

   //重写父类方法 获取屏幕宽高
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        this.screenWidth = getWidth();
        this.screenHeight = getHeight();
        Log.d(TAG, "onSizeChanged:" + this.screenWidth + " "
                + this.screenHeight);
    };


    下面是核心代码都在Callback中
    public boolean tryCaptureView(View childView, int pointId) {
            return true;//默认View都可捕获
    }
    public int clampViewPositionHorizontal(View child, int left, int dx) {
            if (dx < 0) { //这里是向左移动
                direction = 1;
                return left;
            } else if (dx > 0) { //向右边滑动
                direction = 2;
                //最多移动到原始位置
                if (child == current.imageView && left > current.left) {
                    return child.getLeft();
                }
            } 
            return left; // 返回变化过的坐标
    };


    public int clampViewPositionVertical(View child, int top, int dy) {
            return child.getTop();// 上下不滑动，返回默认值
    };

    public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
            super.onViewPositionChanged(changedView, left, top, dx, dy);

            if (changedView == next.imageView) { //当滑动的是next图片时，我们把位移传给当前current的图片
                // 下一张图片暂时不滑动
                changedView.layout(next.left, next.top, next.right, next.bottom);

                int _left = current.imageView.getLeft() + dx;
                int _right = current.imageView.getRight() + dx;
                if (current.imageView.getLeft() > current.left) { // 只能让图片回到原来位置
                    current.imageView.layout(current.left, current.top,current.right, current.bottom);
                } else if (_right >= 0) {
                    // 最多滑倒屏幕最左边，就不滑动了 left=-宽度 或者 right =0
                    current.imageView.layout(_left, current.top, _right,current.bottom);
                }
                if (_right <= 4) { //这里处理的不太好，滑动的时候不能一次性滑倒最边缘处，所以可能多出4个位置px
                    current.imageView.layout(_left + _right, current.top, 0,current.bottom);
                }
            }
            //滑动过程中，next图片动画
            viewDragAnimate(dx, current.right, next);

            // 已经完全滑动到屏幕外边，我们要调换图片
            if (current.imageView.getRight() <= 0) {
                if (!viewreleased) {
                    adjustView(current, next);
                }
            }

            invalidate();
};


//来看动画处理
    float scaleM = defScaleX; // X 和 Y 都一样
    float alphaM = defAlpha;

    private void viewDragAnimate(int dx, int right, Holder change) {
        float distance = right;// 0 - right 为滑动范围
        float factor = (1 - defScaleX) / distance;// 计算每一步扩大因子
        factor = factor * Math.abs(dx);//乘以步伐
        if (dx < 0) { // 扩大
                scaleM += factor;
                alphaM += factor;
        } else if (dx > 0) { // 放小
                scaleM -= factor;
                alphaM -= factor;
        }
            // 检测不越界
        if (scaleM >= 1.0) {
                scaleM = 1.0f;
        }
        if (scaleM <= defScaleX) {
                scaleM = defScaleX;
        }
        if (alphaM >= 1.0) {
                alphaM = 1.0f;
        }
        if (alphaM <= defAlpha) {
                alphaM = defAlpha;
        }
        change.imageView.setScaleX(scaleM);
        change.imageView.setScaleY(scaleM);
        change.imageView.setAlpha(alphaM);

}
private void adjustView(Holder current, Holder next) {
            Log.d(TAG, "adjustView>>>");
            // 交换一下位置,未来实现滑动前一张图片
            Holder temp = current; // CURRENT -> NextCache
            cachenext.imageView.setImageDrawable(temp.imageView.getDrawable());
            cachenext.setPosition(temp.left, temp.top, temp.right, temp.bottom);
          cachenext.imageView.layout(
          temp.imageView.getLeft(),
          temp.imageView.getTop(),  
          temp.imageView.getRight(),
          temp.imageView.getBottom());
          //nextcache考虑未来向右边滑动时

            // NEXT -> CURRENT
            Bitmap bitmap = ((BitmapDrawable) next.imageView.getDrawable()).getBitmap();
            int w = bitmap.getWidth();
            int h = bitmap.getHeight();
            current.imageView.setImageBitmap(bitmap);
            current.left = (screenWidth - w) / 2; // 重新计算图片位置
            current.top = (screenHeight - h) / 2;
            current.right = current.left + w;
            current.bottom = current.top + h;
            FrameLayout.LayoutParams fm = new FrameLayout.LayoutParams(w, h);
            fm.gravity = Gravity.CENTER;// 重新设置 否则可能不会居中
            current.imageView.setLayoutParams(fm);
            current.imageView.layout(current.left, current.top, current.right,current.bottom);


            // 重新开始next图片
            scaleM = defScaleX; // X 和 Y 都一样
            alphaM = defAlpha;
            bitmap = BitmapFactory.decodeResource(getResources(),images[currentIndex++]);
            next.imageView.setImageBitmap(bitmap);
            next.imageView.setScaleX(defScaleX);
            next.imageView.setScaleY(defScaleY);
            next.imageView.setAlpha(defAlpha);

            w = bitmap.getWidth();
            h = bitmap.getHeight();
            next.left = (screenWidth - w) / 2;
            next.top = (screenHeight - h) / 2;
            next.right = next.left + w;
            next.bottom = next.top + h;
            fm = new FrameLayout.LayoutParams(w, h);
            fm.gravity = Gravity.CENTER;
            next.imageView.setLayoutParams(fm);
            next.imageView.layout(next.left, next.top, next.right, next.bottom);


            if (currentIndex >= images.length) {
                currentIndex = 0;
            }
}

最后是动画
public void onViewReleased(View releasedChild, float xvel, float yvel) {
            super.onViewReleased(releasedChild, xvel, yvel);
            viewreleased = true;//滑动过程中 避免修改View 
            resetPositionSmoothly();
};

private void resetPositionSmoothly() {
        if (isSmooth) {
            int destLeft;
            int p = current.left / 2; // 一共能滑动多少，一半以内回到原来地方，一半以外滑动到屏幕外面
            if (current.imageView.getLeft() < p) {
                destLeft = -(current.right - current.left);// 滑倒屏幕外
            } else {
                destLeft = current.left;// 恢复
            }
            //带动画滑动效果
            boolean isok = viewDragHelper.smoothSlideViewTo(current.imageView,destLeft, current.top);
            if (isok) {
                ViewCompat.postInvalidateOnAnimation(this);
            }
        }
}

@Override
public void computeScroll() {
        super.computeScroll();
        if (viewDragHelper.continueSettling(true)) {
            ViewCompat.postInvalidateOnAnimation(this);
        } else {
            viewreleased = false;
        }
}

OK 代码就这些，基本功能实现就是这样的，大部分都是计算坐标，ViewDragHelper让我们减少了很多代码。